In this study, the influence of the negative pressure generated during demolding in the injection molding process on the demolding resistance is clarified by comparing the demolding forces in the cases of demolding molded products with and without holes on the bottom. Further, as an evaluation method of the demolding resistance under vacuum negative pressure, it is proposed to use the overall energy spent in the mold release motions from start to end of the demolding process. Furthermore, a breathable mold, where the porous material is locally incorporated, was designed and manufactured to reduce the negative pressure during demolding process. Finally, the effectivity of reducing the demolding resistance by using the breathable mold was experimentally verified. The experimental results showed a higher reducing effect for demolding resistance could be obtained.